Polysilanes catalysts

Dehydrogenative Coupling of Hydride Functional Silanes. The autocouphng of dihydridosilanes was first observed usiag Wilkinson s catalyst (128). A considerable effort has been undertaken to enhance catalyst turnover and iacrease the molecular weight of polysilane products (129) because the materials have commercial potential ia ceramic, photoresist, and conductive polymer technology. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

These and similar complexes of Ti and Zrare effective catalysts for the formation of polysilanes from primary silanes [117-120]. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

Recently, the compounds CpCp Hf(Cl)Si(SiMe3)3 and CpCp Zr(Cl)Si(SiMe3)3 have been used as homogenous catalysts for the formation of polysilanes. 

The Kumada/Ishikawa group also investigated thermolytic reactions of alkynyl-polysilanes and silacyclopropenes in the presence of nickel catalysts and implicated a 1-silaallene-nickel complex as an intermediate in the reaction pathway... 

A similar dichotomy was observed in the titanium catalyzed polymerization of primary silanes coupled to the hydrogenation of norbornene (20). At low catalyst concentration (ca. 0.004H), essentially complete conversion of norbornene to an equimolar mixture of norbornane and bis-phenylsilyl- (and/or 1,2-diphenyl-disilyl)norbornane was observed. Under these conditions no evidence for reduction of titanium was obtained. At higher catalyst concentrations (> 0.02M) rapid reduction of the dimethyltitanocene to J, and 2 occurs and the catalytic reaction produces mainly polysilane (DPn ca. 10) and norbornane in ca. 80 per cent yields, and silylated norbornanes in about 20 per cent yield. 

The first step for the synthesis of a melt spinnable polysilane is the alkoxylation and distillation of the residue (Figure 1). 1,2-dimethyltetramethoxydisilane and 1,1,2-trimethyltrimethoxydisilane are mixed in a special ratio and a poly silane will be obtained by a catalytic redistribution reaction. Catalysts for this reaction are alkali alkoxides like sodium methoxylate. Phenylmethoxydisilanes [22] or phenylchloride are used as additives. A mixture of methyltrimethoxysilane and dimethyldimethoxy-silane distils off as a byproduct of the redistribution reaction. Figure 2 shows the mechanism of the catalytic redistribution. 

Polysilanes containing phenyl groups are thermally stable up to 350°C while methylmethoxypoly-silanes without phenyl groups decompose at 250 - 300°C. When methylmethoxyphenylpolysilanes are heated to 350°C methylmethoxysilanes distil off and a polymer is formed which is nearly free of methoxy groups. In contrast to the thermal reaction of polydimethylsilanes no polycarbosilane structures can be observed even if polyborodiphenylsiloxane is added as catalyst (Figure 3.). 

Harrod-type catalytic dehydrocoupling method using early transition metal catalysts (see COMC II (1995), chapter Organopolysilanes, p 99, and earlier in this review, Section 3.11.4.1.3. (i)).69,75 Si-H bonds are susceptible to free radical attack, and use of this was made in the free radical substitution of 38 to prepare a number of oxy-functionalized polysilanes, as shown in Scheme 25.185,186... 

Most recently, a simple, mild, one-pot immobilization method was developed to attach the rigid rod-like helical polysilane, poly( -decyl-/-butylsilylene), via a siloxy linkage to hydrophilic quartz or mica substrate surfaces.28 195 Triethylamine was used as a catalyst to couple the Si-H and/or Si-OR termini of the dialkylpolysilane chains (which are generated during the course of Wurtz-type synthesis and workup)51,195 with the surface -OH groups. AFM, UV, and IR data were used to analyze the reactions. 

A manifold of dendrimers have been presented in the literature ranging from polyamidoamine, polyfpropylene imine), aromatic polyether and polyester, aliphatic polyether and polyester, polyalkane,polyphenylene, polysilane, and phosphorus dendrimers. Combinations of different backbones as well as architectural modifications have also been presented. For example, the incorporation of chirality in dendrimers, copolymers of linear blocks with dendrimer segments (dendrons), and block copolymers of different dendrons has been described. Numerous applications have been proposed for dendrimers such as biotemplates, liquid membranes, catalysts, or in medical applications. ... 

Because of these limitations, considerable effort has been focused on the development of new synthetic routes to polysilanes. The early transition metal-catalysed dehydrocoupling process discovered in 1985 [eqn (10.8)] is potentially very attractive however, the molecular weights of the polysilanes formed to date are generally fairly low (Mn<8000)." The catalysts used for these coupling reactions are usually titanocene or zirconocene derivatives." " " ... 

Tanaka and co-workers also reported in 1991 that addition of the unactivated Me3Si-SiMe3 to terminal alkynes is catalyzed by the Pd(dba)2 system in the presence of a particularly small basic phosphine ligand, P(OCH2)3-CEt.60 Using this catalyst, it is also possible to insert acetylene into multiple Si-Si bonds in polysilanes and polycarbodisilanes [Eq. (17)]. 

Carbon functional groups, attachment to polysilanes, 3, 585 Carbon-heteroatom bond formation via antimony(III) compounds, 9, 428 via antimony(V) compounds, 9, 432 via bismuth(III) compounds characteristics, 9, 440 with copper catalysts, 9, 442 non-catalyzed reactions, 9, 443 with bismuth(V) compounds, 9, 450 with bismuthonium salts, 9, 449 with bismuth ylides, 9, 450 Carbon-heteroatom ligands in tetraosmium clusters, 6, 967 in tetraruthenium clusters, 6, 960... 

Lactones, via indium compounds, 9, 686 Lactonizations, via ruthenium catalysts, 10, 160 Ladder polysilanes, preparation and properties, 3, 639 Lanthanacarboranes, synthesis, 3, 249 Lanthanide complexes with alkenyls, 4, 17 with alkyls, 4, 7 with alkynyls, 4, 17 with allyls, 4, 19 with arenes, 4, 119, 4, 118 and aromatic C-F bond activation, 1, 738 bis(Cp ), 4, 73... 

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